Electrode for power storage device and power storage device

ABSTRACT

An electrode for a power storage device with good cycle characteristics and high charge/discharge capacity is provided. In addition, a power storage device including the electrode is provided. The electrode for the power storage device includes a conductive layer and an active material layer provided over the conductive layer, the active material layer includes graphene and an active material including a plurality of whiskers, and the graphene is provided to be attached to a surface portion of the active material including a plurality of whiskers and to have holes in part of the active material layer. Further, in the electrode for the power storage device, the graphene is provided to be attached to a surface portion of the active material including a plurality of whiskers and to cover the active material including a plurality of whiskers. Further, the power storage device including the electrode is manufactured.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electrode for a power storage deviceand a power storage device including the electrode.

Note that the power storage device indicates all elements and deviceswhich have a function of storing power.

2. Description of the Related Art

In recent years, power storage devices such as lithium-ion secondarybatteries, lithium-ion capacitors, and air cells have been developed.

An electrode for a power storage device is manufactured by forming anactive material layer over one surface of a current collector. As theactive material, a material which can occlude and release ions, such ascarbon or silicon, is used. In addition, phosphorus-doped silicon haslarger theoretical capacity than carbon and thus is advantageous interms of increasing capacity of a power storage device (see PatentDocument 1).

Owing to excellent electric characteristics such as high conductivity(high electron mobility) and physical characteristics such asflexibility and mechanical strength, application of graphene to avariety of products has been attempted (see Patent Documents 2 and 3).In addition, a technique for applying graphene to a lithium-ionsecondary battery has been proposed (see Patent Document 4).

REFERENCE Patent Document [Patent Document 1] Japanese Published PatentApplication No.2001-210315 [Patent Document 2] United States PublishedPatent Application No. 2009/0110627 [Patent Document 3] United StatesPublished Patent Application No. 2007/0131915 [Patent Document 4] UnitedStates Published Patent Application No. 2010/0081057 SUMMARY OF THEINVENTION

When silicon is used as an active material layer of an electrode for apower storage device, the silicon expands and contracts repeatedly dueto repeated charge/discharge cycles. As a result, the silicon which isused as an active material layer turns into fine powder and separationor the like is caused, which deteriorates properties of the powerstorage device.

Further, even when silicon is used as an active material layer, it isdifficult to obtain charge/discharge capacity as high as the theoreticalcapacity.

Thus, an object of one embodiment of the present invention is to providean electrode for a power storage device with good cycle characteristicsand high charge/discharge capacity. In addition, another object is toprovide a power storage device including the electrode.

One embodiment of the present invention is an electrode for a powerstorage device which includes a conductive layer and an active materiallayer provided over the conductive layer, and in which the activematerial layer includes graphene and an active material including aplurality of whiskers, and the graphene is provided to be attached to asurface portion of the active material including a plurality of whiskersand to have holes in part of the active material layer.

Another embodiment of the present invention is an electrode for a powerstorage device, in which the active material layer includes graphene andthe active material including a plurality of whiskers and the grapheneis provided to be attached to a surface portion of the active materialincluding a plurality of whiskers and to cover the active materialincluding a plurality of whiskers. Further, the graphene is provided tospread continuously over the active material including a plurality ofwhiskers in a plan view of the active material layer.

In any of the above structures, the active material including aplurality of whiskers includes at least a core which has a structurehaving crystallinity and an outer shell which is provided to cover thecore and has an amorphous structure.

Further, in any of the above structures, a material of the activematerial including a plurality of whiskers is silicon, for example.

Furthermore, in any of the above structures, a material of theconductive layer is preferably titanium, zirconium, hafnium, vanadium,niobium, tantalum, chromium, molybdenum, tungsten, cobalt, or nickel.

Still another embodiment of the present invention is a power storagedevice including the electrode having any of the above structures.

In the electrode for a power storage device which is an embodiment ofthe present invention, even if the volume of the active materialincluding a whisker is changed due to occlusion and release of ions, thegraphene relieves stress caused by the change in volume, so that thestructure of the electrode is not easily damaged, for example,pulverization and separation of the active material including a whisker.According to one embodiment of the present invention, an electrode for apower storage device which can improve cycle characteristics can beprovided, and in addition, by including the electrode, a power storagedevice with improved cycle characteristics can be provided.

Further, the electrode for a power storage device which is an embodimentof the present invention has excellent electric characteristics,because, for example, a core which has a structure having crystallinityis provided in the active material including a plurality of whiskers,and graphene which has high conductivity (high electron mobility) isprovided between the active material including a plurality of whiskers.According to one embodiment of the present invention, an electrode for apower storage device which can improve charge/discharge capacity can beprovided, and in addition, by including the electrode, a power storagedevice with improved charge/discharge capacity can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B are schematic cross-sectional views each illustrating asurface of an electrode for a power storage device according to oneembodiment of the present invention;

FIG. 2 is a cross-sectional view for illustrating an electrophoresismethod;

FIGS. 3A and 3B are a plan view and a cross-sectional view illustratingone embodiment of a power storage device;

FIG. 4 is a view illustrating application examples of a power storagedevice;

FIGS. 5A and 5B are plan SEM images of a surface of an electrode for apower storage device according to one embodiment of the presentinvention;

FIGS. 6A and 6B are cross-sectional TEM images of a surface of anelectrode for a power storage device according to one embodiment of thepresent invention;

FIGS. 7A and 7B are cross-sectional TEM images of a surface of anelectrode for a power storage device according to one embodiment of thepresent invention;

FIG. 8 is a perspective view illustrating a manufacturing method of apower storage device according to one embodiment of the presentinvention;

FIGS. 9A and 9B are graphs showing cycle characteristics andcharge/discharge characteristics of a power storage device according toone embodiment of the present invention; and

FIGS. 10A and 10B are schematic cross-sectional views each illustratinga surface of an electrode for a power storage device according to oneembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments and examples of the present invention will be describedhereinafter with reference to the drawings. Note that the invention isnot limited to the following description, and it will be easilyunderstood by those skilled in the art that various changes andmodifications can be made without departing from the spirit and scope ofthe invention. Thus, the present invention should not be interpreted asbeing limited to the following description of the embodiments. Indescription with reference to the drawings, in some cases, the samereference numerals are used in common for the same portions in differentdrawings. Further, in some cases, the same hatching patterns are appliedto similar parts, and the similar parts are not necessarily designatedby reference numerals.

Embodiment 1

In this embodiment, an electrode according to an embodiment of thepresent invention and a method for manufacturing the electrode will bedescribed with reference to the drawings.

FIGS. 1A and 1B are schematic cross-sectional views illustrating part ofa surface of an electrode 100 according to one embodiment of the presentinvention. The electrode 100 includes a substrate 101, a conductivelayer 103 provided over the substrate 101, and an active material layer108 provided over the conductive layer 103.

The active material layer 108 includes an active material including aplurality of whiskers including a region 107 a and a region 107 b, andgraphene 116 including a first region 113 and a second region 115 andbeing in contact with the active material including a plurality ofwhiskers. Note that in this specification, an active material includinga whisker refers to an active material including a flat region such asthe region 107 a and a projecting region from the region 107 a like awhisker (or like a string or a fiber) such as the region 107 b. Further,in order to clearly show that the active material layer 108 includes aplurality of whiskers each of which is a projecting region of the activematerial, as illustrated in FIGS. 1A and 1B, the active materialincluding the region 107 a and the region 107 b is referred to as theactive material including a plurality of whiskers.

The region 107 a is provided in contact with the conductive layer 103and the region 107 b projects from the region 107 a and is provided tobe dispersed at random. Therefore, the active material layer 108 has afine surface structure reflecting the shapes of the active materialincluding a plurality of whiskers.

Further, a mixed layer 105 may be provided in a part of or the whole ofa surface layer of the conductive layer 103 by reaction with the activematerial layer 108 (in particular, the active material including aplurality of whiskers). Note that the mixed layer 105 also functions asa conductive layer because the mixed layer 105 has conductivity. In thecase where the mixed layer 105 is formed on the part of the surfacelayer of the conductive layer 103, the mixed layer 105 and the part ofthe conductive layer 103 are provided below the active materialincluding a plurality of whiskers (in particular, the region 107 a) (notillustrated). In the case where the mixed layer 105 is formed on thewhole of the surface layer of the conductive layer 103, the mixed layer105 is provided below the active material including a plurality ofwhiskers (in particular, the region 107 a) (see FIGS. 1A and 1B).

Note that the interface between the region 107 a and the region 107 b isnot clear. Thus, the following surface is defined as the interfacebetween the region 107 a and the region 107 b: a surface which is incontact with a bottom of the lowest hollow of hollows each formed in theactive material layer 108 (between the active materials including aplurality of whiskers) and parallel to a surface of the substrate 101 orthe conductive layer 103.

In the active material layer 108, the active material including aplurality of whiskers preferably includes a core 109 which has astructure having crystallinity and an outer shell 111 which has anamorphous structure. The amorphous structure has a feature of resistanceto change in volume due to occlusion and release of ions (e.g., stresscaused by change in volume is relieved). The structure havingcrystallinity has an excellent conductivity and ion mobility and has afeature of high speeds of occlusion and release of ions per unit mass.Therefore, by using the electrode 100 including the active materialincluding a plurality of whiskers including the core 109 and the outershell 111, output characteristics are improved, and a power storagedevice with high charge/discharge capacity and improved cyclecharacteristics can be manufactured.

Note that the core 109 is not limited to the core which is in contactwith the conductive layer 103 such as a core 109 a, and may be the corewhich extends from front to back of the drawing such as a core 109 b andthe core which is localized such as a core 109 c. That is, the core 109a, the core 109 b, and the core 109 c are collectively referred to asthe core 109. Further, an outer shell 111 a, an outer shell 111 b, andan outer shell 111 c are collectively referred to as the outer shell111.

The region 107 b may each have a columnar (cylindrical or prismatic)shape or a conical or pyramidal shape (which may also be referred to asneedle-like shape). Further, a top of the active material including aplurality of whiskers may be curved.

The longitudinal directions of the active material including a pluralityof whiskers do not necessarily have to be the same. When thelongitudinal directions of the active material including a whisker aredifferent from each other, in FIGS. 1A and 1B, a transversecross-sectional shape of the active material (the cross-sectional shapeof a portion including the core 109 a and the outer shell 111 a) isshown as well as a longitudinal cross-sectional shape of the activematerial (the cross-sectional shape of a portion including the core 109b and the outer shell 111 b). In a transverse cross section of theactive material including a whisker, the core 109 is observed (is notobserved) in the active material including a whisker in some casesdepending on a position. Further, the transverse cross section of theactive material including a whisker is circular when the active materialincluding a whisker has a cylindrical or conical shape, and is polygonalwhen the active material including a whisker has a prismatic orpyramidal shape. It is preferable that the longitudinal directions ofthe active material including a whisker be not the same because one ofthe active material including a whisker is likely to be entangled withthe other, so that separation (or detachment) of the active materialincluding a whisker does not easily occur in charging/discharging.

Note that the direction in which the active material including a whiskeris extended from the region 107 a is called the longitudinal direction,and the cross-sectional shape of the active material including a whiskercut along the longitudinal direction is called a longitudinalcross-sectional shape. In addition, a cross-sectional shape of theactive material including a whisker cut along a surface perpendicular toor substantially perpendicular to the longitudinal direction of theactive material including a whisker is called a transversecross-sectional shape.

The width of the core 109 in a transverse cross-sectional shape isgreater than or equal to 0.2 μm and less than or equal to 3 μm,preferably greater than or equal to 0.5 μm and less than or equal to 2μm.

The length of each of the cores 109 is not particularly limited but maybe greater than or equal to 0.5 μm and less than or equal to 1000 μm,preferably greater than or equal to 2.5 μm and less than or equal to 100μm.

In the region 107 b, the width of the transverse cross section of theactive material including a plurality of whiskers is greater than orequal to 0.2 μm and less than or equal to 10 μm, preferably greater thanor equal to 1 μm and less than or equal to 5 μm. The length of each ofthe active materials including a plurality of whiskers is greater thanor equal to 3 μm and less than or equal to 1000 μm, preferably greaterthan or equal to 6 μm and less than or equal to 200 μm.

Note that in the longitudinal cross-sectional shape of the activematerial including a whisker, the “length” of the core 109 and the outershell 111 refers to the distance between a vertex of the core 109 or theouter shell 111 and the region 107 a, in the direction along an axispassing through the center of the vertex (or the top surface) of theactive material including a whisker.

Further, the structure of the active material including a plurality ofwhiskers is not limited to the described-above structure; the whole ofthe region 107 a and the region 107 b may have a structure havingcrystallinity, and the whole of the region 107 a and the region 107 bmay have an amorphous structure (e.g., the outer shell 111 c).

In the electrode 100 illustrated in FIG. 1A, part of the region 107 a (aregion other than the region where the conductive layer 103 is incontact with the core 109) has an amorphous structure like the outershell 111. Further, the region 107 a may include a structure havingcrystallinity. Furthermore, the region 107 a may include one or both ofthe materials of the conductive layer 103 and a material of the mixedlayer 105.

Moreover, in the electrode 100, as illustrated in FIG. 1B, a portion ofthe region 107 a which is in contact with the conductive layer 103 mayhave a structure having crystallinity like the core 109. Further, theregion 107 a may include an amorphous structure. Furthermore, the region107 a may include one or both of the materials of the conductive layer103 and the material of the mixed layer 105.

For example, in the case where the electrode 100 is the mode illustratedin FIG. 1A, adhesion between the conductive layer 103 and the region 107a is higher than in the case where the electrode 100 is the modeillustrated in FIG. 1B. This is because an amorphous structure is moreadaptable to the surface of the conductive layer 103, over which theactive material is formed. Moreover, in the case where this embodimentis included in the power storage device, the active material isresistant to change in volume due to occlusion and release of ions(e.g., the active material which has an amorphous structure relievesstress caused by change in volume), so that pulverization and separationof the electrode 100 (in particular, the active material including awhisker) due to repeating charge/discharge cycles can be prevented, anda power storage device having much higher cycle characteristics can bemanufactured.

Alternatively, in the case where the electrode 100 is the modeillustrated in FIG. 1B, a structure having crystallinity which hashigher conductivity and ion mobility is in contact with the conductivelayer 103 wider than that in the case where the electrode 100 is themode illustrated in FIG. 1A. Therefore, as the whole electrode 100, theconductivity can be increased. That is, in the case where thisembodiment is included in the power storage device, a power storagedevice with much higher output characteristics can be manufactured.

In the electrode 100, the surface area of the active material layer 108is larger than that of a plate-like active material layer due to theprojecting region 107 b. Moreover, since the graphene 116 is included inthe active material layer 108, the surface area of the active materiallayer 108 is much larger. That is, in the case where the electrode 100is included in a power storage device, high speed charge/discharge canbe performed, so that a power storage device with much highercharge/discharge capacity can be manufactured.

As illustrated in FIGS. 1A and 1B, the active material layer 108 in theelectrode 100 includes the graphene 116 which is in contact with theactive material including a plurality of whiskers and the graphene 116includes the first region 113 and the second region 115.

In this specification, graphene refers to a one-atom-thick sheet ofcarbon molecules having a hole through which ions can pass and sp²bonds, or a stacked body in which 2 to 100 layers of the sheets arestacked. Note that the stacked body is also referred to as a multilayergraphene. Further, in the graphene, the proportion of the elementsexcept hydrogen and carbon is preferably lower than or equal to 15atomic %, or the proportion of elements except carbon is preferablylower than or equal to 30 atomic %. This is because an analog ofgraphene is included in the category of the graphene.

The first region 113 covers each of the active materials including aplurality of whiskers. In detail, the first region 113 also covers asurface of the region 107 a as well as a surface of the active materialin the projecting region 107 b. Further, the thickness of the firstregion 113 is not uniform in the whole area and may be uneven.

A second region 115 is provided between a first region 113 at a side ofone of the active materials including whiskers and at least one of otherfirst regions 113 at a side of an active material. Moreover, the secondregions 115 dot the region where the active material including aplurality of whiskers is formed. Therefore, in a plan view of the activematerial layer 108, there are holes (also referred to as spaces) atintervals (not illustrated) in the active material layer 108. Further,the thickness of the second region 115 is not uniform in the whole areaand may be uneven.

Note that in the graphene 116, the interface between the first region113 and the second region 115 is not clear. In FIGS. 1A and 1B, in aregion where the first region 113 is bonded to the second region 115,the first region 113 and the second region 115 are individuallyillustrated by dotted lines for clarity.

As illustrated in dashed dotted lines in FIGS. 1A and 1B, the secondregion 115 in the graphene 116 may be provided between a first region113 at a side of one of the active materials including a whisker and afirst region 113 at a surface of the region 107 a. The second region 115illustrated in FIGS. 1A and 1B extends perpendicularly to the substrate101; however, the second region 115 is not limited thereto and may be along and narrow string shape which is attached to a first region 113 ata side of one of the active materials including a whisker (notillustrated).

Further, the active material layer 108 may have a structure in which anoxide film is provided between the first region 113 and the activematerial including a plurality of whiskers. However, in the terms ofconductivity of the electrode 100, the oxide film is preferably notprovided.

The graphene 116 is highly flexible and in contact with the activematerial including a plurality of whiskers. Therefore, in the electrode100 in which the graphene 116 is included in the active material layer108, even if the volume of the active material including a plurality ofwhiskers is changed due to occlusion and release of ions, the graphene116 relieves stress caused by the change in volume, so thatpulverization and separation of the active material including a whiskerdue to repeating charge/discharge cycles can be prevented. Moreover, thegraphene 116 has high mechanical strength; therefore, bending andcollapsing (also pulverization and separation) of the active materialincluding a whisker by physical impact in the electrode 100 includingthe graphene 116 can be prevented. Therefore, by using the electrode100, a decrease in charge/discharge capacity by physical impact orrepeating charge/discharge cycles can be suppressed, so that a powerstorage device having higher cycle characteristics can be manufactured.

Further, the graphene 116 has high conductivity (electron mobility) andis in contact with the active material including a plurality ofwhiskers, whereby in the electrode 100, the graphene 116 can function asa conduction auxiliary agent. That is, the graphene 116 becomes aconduction path of electrons generated due to occlusion and release ofions. This is because the electrode 100 is superior in conductivity.Therefore, by using the electrode 100, high speed charge/discharge canbe performed, and a power storage device with higher charge/dischargecapacity can be manufactured.

Further, the graphene 116 is in contact with the active materialincluding a plurality of whiskers of the active material layer 108,whereby, for example, even when the active material including a whiskeris bent or collapsed (also pulverized and separated), the state wherethe active material including a whisker are in contact with the graphene116 is maintained, so that the conduction path of electrons in theelectrode 100 is not interrupted and electrons can be collected throughthe graphene 116. That is, even when the active material including awhisker is bent or collapsed (also pulverized and separated), decreasein conductivity between the conductive layer 103 and the active materiallayer 108 (consequently conductivity of the electrode 100) can besuppressed.

Further, because the graphene 116 itself has a capacity so that ions canbe occluded, the electrode 100 with the graphene 116 functions as anelectrode with a higher capacity than the electrode 100 without thegraphene 116. Therefore, by using the electrode 100, a power storagedevice with higher charge/discharge capacity can be manufactured.

Further, in the electrode 100, the graphene 116 is in contact with theactive material including a plurality of whiskers, whereby it can besaid that the graphene 116 ties the active material including aplurality of whiskers. That is, the graphene 116 also functions as abinder. As described above, the electrode 100 is formed without a knownconduction auxiliary agent (such as acetylene black) or a binder. As aresult, the electrode 100 can increase the volume or weight proportionof the active material layer 108 in the electrode, so that the electrode100 functions as an electrode with high capacity. Therefore, using theelectrode 100, a power storage device with higher charge/dischargecapacity can be manufactured.

Further, the graphene 116 also has high heat resistance. Therefore, inthe electrode 100, the concentration of moisture in the electrode can bereduced by heat treatment. Moreover, in the case where the electrode 100is included in the power storage device, the graphene 116 of theelectrode 100 is less capable of absorbing an electrolyte, so thatdeformation or breakdown of the electrode 100 due to swelling of thegraphene 116 hardly occurs.

<Method for Manufacturing Electrode 100>

Next, a method for manufacturing the electrode 100 according to oneembodiment of the present invention will be described.

The conductive layer 103 is formed over the substrate 101. Theconductive layer 103 can be formed using a conductive material and amethod such as a printing method, a sol-gel method, a coating method, anink-jet method, a CVD method, a sputtering method, or an evaporationmethod as appropriate. In addition, the conductive layer 103 may beformed in a foil shape, a plate shape, a net shape, or the like. Notethat in the case where the conductive layer 103 has a foil shape or aplate shape, it is not necessary to provide the substrate 101. Further,the conductive layer 103 can be formed through roll-to-roll processing.

As described below, depending on the formation conditions of the activematerial layer 108, the mixed layer 105 may be formed on a part of thesurface layer or the whole of the surface layer of the conductive layer103 in some cases.

Further, over the substrate 101, the conductive layer 103 may have astacked layer which includes a layer formed using a metal materialhaving high conductivity typified by an aluminum alloy and the like towhich an element which improves heat resistance is added; and over thelayer, a layer formed using a metal material which forms the mixed layer105. Note that as the element which improves heat resistance, forexample, platinum, aluminum, copper, silicon, titanium, neodymium,scandium, molybdenum, or the like can be used.

Next, the active material layer 108 is formed over the conductive layer103. First, the active material including a plurality of whiskers isformed. There is no particular limitation on a material of the activematerial including a plurality of whiskers as long as it can be formedlike the plurality of whiskers and it can occlude and release ions. Forexample, silicon can be used as the material, and in this embodiment, amethod for manufacturing the electrode 100 in the case where silicon isused as the material is described.

The active material including a plurality of whiskers s can be formed byan LPCVD (low pressure CVD) method. Here, the active material includinga plurality of whiskers is preferably formed at a temperature higherthan 400° C. and lower than or equal to a temperature which an LPCVDapparatus, the substrate 101, and the conductive layer 103 canwithstand, and preferably higher than or equal to 500° C. and lower than580° C. Note that in the case of the mode illustrated in FIG 1A, theupper limit of the temperature range is set to be less than temperaturesat which silicon of the active material including a plurality ofwhiskers does not become amorphous.

In the case where the active material including a plurality of whiskersis formed, as a source gas, a deposition gas containing silicon is used.As the deposition gas containing silicon, a gas containing siliconhydride, silicon fluoride or silicon chloride can be used. Specifically,SiH₄, Si₂H₆, SiF₄, SiCl₄, Si₂Cl₆, or the like can be used. Note that oneor more of a hydrogen gas and rare gases such as helium, neon, argon,and xenon may be contained in the source gas.

Furthermore, in the formation of the active material including aplurality of whiskers, the pressure is set to be higher than or equal to10 Pa and lower than or equal to 1000 Pa, preferably higher than orequal to 20 Pa and lower than or equal to 200 Pa. However, in the caseof the embodiment illustrated in FIG. 1A, the pressure is set to withinthe range in which silicon in the active material including a pluralityof whiskers has an amorphous structure, and in the case of theembodiment illustrated in FIG. 1B, the pressure falls within the rangein which silicon in the active material including a plurality ofwhiskers has a structure having crystallinity.

Further, when the flow rate of the deposition gas containing silicon ishigh, the deposition rate becomes high, so that silicon of the activematerial including a plurality of whiskers is likely to have anamorphous structure. When the flow rate of the deposition gas containingsilicon is low, the deposition rate becomes low, so that silicon of theactive material including a plurality of whiskers is likely to have astructure having crystallinity. Thus, the flow rate of the depositiongas containing silicon may be appropriately determined in considerationof the deposition rate and the like. For example, the flow rate of thedeposition gas containing silicon may be greater than or equal to 300sccm and less than or equal to 1000 sccm.

When the source gas contains phosphine or diborane, the active materialincluding a plurality of whiskers can contain an impurity elementimparting one conductivity type (e.g., phosphorus or boron). When theactive material including a plurality of whiskers contains an impurityelement imparting one conductivity type, the conductivity of theelectrode 100 can be increased, so that a power storage device in whichcharge/discharge capacity is increased can be manufactured.

Further, the embodiment illustrated in FIG. 1B can be easilymanufactured by performing the formation of the active materials by anLPCVD method in two installments. After an active material is formedonce, heat treatment is performed, and the other active material isformed after the heat treatment. By the heat treatment, the whole regionof the region 107 a can have a structure having crystallinity. Note thatthe formation conditions of the active material is as described-above,the heat treatment may be performed in the temperature range in theformation conditions of the active material, preferably performed in thestate where the source gas is not supplied.

Here, in the case where the mixed layer 105 is formed, the material ofthe active material including a plurality of whiskers is silicon,whereby silicide is formed in the mixed layer 105.

The mixed layer 105 is formed in such a manner that active species inthe source gas (e.g., radicals which are derived from the depositiongas, or hydrogen radicals) are supplied to a surface of the conductivelayer 103, so that silicon is diffused into the conductive layer 103from the active material including a plurality of whiskers. As the metalmaterial which forms silicide, titanium, zirconium, hafnium, vanadium,niobium, tantalum, chromium, molybdenum, cobalt, nickel, or the like canbe used. Thus, as the material of the conductive layer 103, the metalmaterials listed above can be used.

Note that the conductive layer 103 (or the mixed layer 105) may have anuneven shape in advance. In this manner, formation density of the activematerial including a plurality of whiskers (in particular, the region107 b) per unit area can be increased. In order that the conductivelayer 103 can have an uneven shape, the conductive layer 103 may besubjected to a photolithography process and an etching step. Further,when the conductive layer 103 is formed thin (to a thickness ofapproximately 2 nm to 100 nm for example), the conductive layer 103 canhave an uneven shape reflecting roughness of a surface (the substrate101) over which the conductive layer 103 is formed.

Note that, in some cases, a metal oxide layer (not illustrated) formedof an oxide of a metal material contained in the conductive layer 103 isformed over the conductive layer 103. This is because oxygen is releasedfrom a sidewall of a quartz reaction chamber of the LPCVD apparatus dueto the heating in forming the active material including a plurality ofwhiskers by an LPCVD method and the conductive layer 103 is oxidized. Atthis time, the reaction chamber may be filled with a rare gas such ashelium, neon, argon, xenon or the like, whereby the formation of themetal oxide layer can be suppressed. Also in the case where the mixedlayer 105 is formed, a metal oxide layer formed of an oxide of a metalmaterial contained in the mixed layer 105 is formed on the mixed layer105 in some cases for the above reason. Note that the metal oxide layermay be formed on the surface of the conductive layer 103 beforeformation of the mixed layer 105.

Examples of the above metal oxide layer include zirconium oxide,titanium oxide, hafnium oxide, vanadium oxide, niobium oxide, tantalumoxide, chromium oxide, molybdenum oxide, tungsten oxide, cobalt oxide,nickel oxide, and the like. When the conductive layer 103 is formedusing titanium, zirconium, niobium, tungsten, or the like, the metaloxide layer contains an oxide semiconductor such as titanium oxide,zirconium oxide, niobium oxide, or tungsten oxide; therefore, it ispossible to reduce at least one of resistance (also referred to ascontact resistance) between the conductive layer 103 and the mixed layer105 and resistance between the conductive layer 103 (or the mixed layer105) and the active material including a plurality of whiskers, so thatthe conductivity of the electrode 100 can be increased.

With the LPCVD method, ions and electrons transfer easily at theinterface between the conductive layer 103 and the active material layer108 (in particular, the region 107 a) and the adhesion can be increased.In addition, throughput can be improved.

Next, the graphene 116 which is in contact with the active materialincluding a plurality of whiskers is formed.

First, a graphite oxide solution in which graphite oxide obtained byoxidizing graphite is dispersed is prepared. In this embodiment, thegraphite oxide is formed by an oxidation method called a Hummers method.The Hummers method is as follows. A sulfuric acid solution of potassiumpermanganate or the like is mixed into graphite powder to causeoxidation reaction; thus, a graphite oxide solution is formed. Graphiteoxide contains a functional group such as an epoxy group, a carbonylgroup, a carboxyl group, or a hydroxyl group due to oxidation of carbonin graphite. Accordingly, the interlayer distance of graphite oxide islonger than that of graphite. Then, ultrasonic vibration is transferredto the graphite oxide solution, so that the graphite oxide whoseinterlayer distance is long can be cleaved to give the solution in whichthe graphene oxide is dispersed (a graphene oxide solution), and thesolvent is removed, whereby the graphene oxide is obtained.

Graphene oxide is dispersed in a solvent such as water orN-methylpyrrolidone (NMP), so that the graphene oxide solution isobtained. The solvent is preferably a polar solvent. The concentrationof graphene oxide may be 0.1 g to 10 g per liter. In a solution havingpolarity, different graphene oxides are easily dispersed each otherbecause these substituents have high polarity, in particular, grapheneoxide contains an epoxy group, a carbonyl group, a carboxyl group, ahydroxyl group, or the like. Further, a solution in which commercialgraphene oxide is dispersed in a solvent or a commercial graphene oxidesolution can be used. The length of one side (also referred to as aflake size) of graphene oxide which is used may be preferably less thanor equal to 10 μm.

Next, the graphene oxide solution is applied to the active materialincluding a plurality of whiskers. As a method of applying the grapheneoxide solution to the active material including a plurality of whiskers,a coating method, a spin coating method, a dip coating method, a spraymethod, an electrophoresis method, or the like may be employed.Alternatively, these methods may be combined as appropriate to beemployed. For example, after the graphene oxide solution is applied to abase by a dip coating method, the base is rotated as in a spin coatingmethod, so that the evenness of the thickness of the graphene oxidesolution can be improved.

Thus, in the case where an active material having a complex curvedsurface or unevenness such as the active material including a pluralityof whiskers is provided with the graphene oxide, an electrophoresismethod is preferably used. Here, the case of using an electrophoresismethod will be described below.

FIG. 2 is a cross-sectional view illustrating an electrophoresis method.In a container 201, the solution in which graphene oxide is dispersedand which is obtained by the above method (hereinafter, referred to as agraphene oxide solution 202) is contained. Further, a formation subject203 is put in the graphene oxide solution 202 and is used as an anode.In addition, a conductor 204 serving as a cathode is put in the grapheneoxide solution 202. Note that the formation subject 203 refers to theactive material including a plurality of whiskers including thesubstrate 101 and the conductive layer 103. Further, the conductor 204may be formed using a conductive material, for example, any of the metalmaterials and the alloy materials.

By applying appropriate voltage (e.g., 0.5 V to 20 V) between the anodeand the cathode, a graphene oxide layer is formed over a surface of theformation subject 203, that is, the surface of the active materialincluding a plurality of whiskers. This is because the graphene oxide isnegatively charged in the polar solvent as described above, so that byapplying voltage, the graphene oxide which is negatively charged isdrawn to the anode and deposited on the formation subject 203. Note thatthe voltage which is applied is not necessarily constant. Further, bymeasuring the amount of charge flowing between the anode and thecathode, the thickness of a graphene oxide layer deposited on the objectcan be estimated.

When the graphene oxide layer with a necessary thickness is obtained,the formation subject 203 is taken out of the graphene oxide solution202 and dried.

When a graphene oxide layer is formed on the surface of the formationsubject 203 by an electrophoresis method, graphene oxide is hardlystacked over a portion of the formation subject 203, which is alreadycovered with graphene oxide, due to sufficiently low conductivity ofgraphene oxide. However, the graphene oxide is stacked preferentiallyover a portion which is not yet covered with graphene oxide, whereby thethickness of the graphene oxide formed over the surface of the formationsubject 203 is substantially uniform. Note that the graphene oxideformed over the surface of the formation subject 203 becomes the firstregion 113 of the electrode 100 by reduction treatment described below.

Moreover, when electrophoresis is performed for a longer time than timerequired for covering the surface of the formation subject 203 withgraphene oxide, the graphene oxide which has already covered the surfaceof the formation subject 203 and other graphene oxide which is dispersedin the graphene oxide solution 202 repel each other. As a result, thegraphene oxide is not extended and does not grow so as to cover thesurface of the active material including a plurality of whiskers, butthe graphene oxide is extended and grows like the second region 115 ofthe electrode 100. That is, the graphene oxide is formed between a firstregion 113 at a side of one of the active materials including whiskersand another or other first regions 113 at a side of an active material.The graphene oxide which is extended and grows becomes the second region115 of the electrode 100 by reduction treatment described below.

Time for performing electrophoresis (time for applying voltage) ispreferably longer than time for covering the surface of the formationsubject 203 with the graphene oxide, for example, longer than or equalto 0.5 minutes and shorter than or equal to 30 minutes, more preferablylonger than or equal to 5 minutes and shorter than or equal to 20minutes.

Then, part of oxygen is released from the graphene oxide by reductiontreatment. As reduction treatment, the graphene oxide is heated at 150°C. or higher, preferably 200° C. or higher in a vacuum, in the air, orin a reducing atmosphere such as an inert gas (nitrogen, a rare gas, orthe like) atmosphere. By being heated at a higher temperature and for alonger time, graphene oxide is reduced to a higher extent so thatgraphene 116 with high purity (i.e., with a low concentration ofelements other than carbon) can be obtained. The temperature for heatinghas to be determined in consideration of reactivity between the grapheneoxide and the formation subject 203. Note that graphene oxide is knownto be reduced at 150° C.

Further, when heat treatment is performed at a higher temperature andfor a longer time, more defects are repaired and the conductivity isimproved. From the inventors' measurements, for example, the grapheneoxide over a glass substrate is heated and reduced to be graphene, sothat resistivity of the graphene is approximately 240 MΩcm at a heatingtemperature of 100° C. (for one hour), approximately 4 kΩcm at a heatingtemperature of 200° C. (for one hour), and approximately 2.8 Ωcm at aheating temperature of 300° C. (for one hour). Note that eachresistivity is an average value of eight samples measured by the Van derPauw method.

Since graphite is treated with a sulfuric acid solution of potassiumpermanganate according to the Hummers method, a functional group such asa sulfone group is also bonded to the graphite oxide, and the release(decomposition) of the functional group is performed at higher than orequal to 200° C. and lower than or equal to 300° C., preferably higherthan or equal to 200° C. and lower than or equal to 250° C. Therefore,reduction treatment is preferably performed at higher than or equal to200° C.

Depending on the temperature of reduction treatment, the conductivity ofthe graphene changes as described above; the same can also apply to itsflexibility, strength, and the like. The temperature of the reductiontreatment may be determined in accordance with the requiredconductivity, flexibility, strength, and the like. In the case where theconductivity of graphene used as a binder is not sufficiently high, aknown conduction auxiliary agent (such as acetylene black) is preferablyadded at a required amount so as to compensate the conductivity.

Through the above reduction treatment, the formed graphene oxide becomesthe graphene 116. At that time, in the graphene 116, adjacent graphenesare bonded to form a larger net-like or sheet-like network, so that thefirst region 113 and the second region 115 are formed. In particular,the first region 113 is formed with a substantially uniform thicknesseven at the surface of the active material having a complex curvedsurface or unevenness. Further, graphene oxide having an extremely highflake aspect ratio forms the second region 115 having a long and narrowstring shape through the above reduction treatment.

Therefore, the first region 113 is in contact with the second region 115in any range of the active material layer 108, and holes (also referredto as spaces) at intervals in the active material including a pluralityof whiskers are formed in the plan view of the active material layer108.

Further, the second region 115 of the electrode 100 can be formed byonly a dip coating method. However, with a dip coating method, theactive material with the surface having a complex curved surface orunevenness may fail to be efficiently soaked in the graphene oxidesolution. Thus, after the first region 113 is formed by performing theabove electrophoresis method for a short time (e.g., about 0.5 minutes),a dip coating method is performed, whereby the electrode 100 can bemanufactured more efficiently than the case of only a dip coating methodis performed.

Through the above steps, the active material layer 108 can be formed. Itis preferable that steps from the step of forming the active materialincluding a plurality of whiskers to the step of forming the graphene116 (in particular, the step of forming the graphene oxide) be performedin an atmosphere in which the surface of the active material including aplurality of whiskers is not oxidized. This is because the oxide filmmay be formed between the active materials including a plurality ofwhiskers and the first region 113 and decreases the conductivity of theelectrode 100.

Through the above steps, an electrode for a power storage device withgood cycle characteristics and high charge/discharge capacity can bemanufactured.

Although the electrode according to one embodiment of the presentinvention is shown as the electrode for a power storage device in theabove, the electrode according to one embodiment of the presentinvention may be used for another application. For example, an activematerial layer of the electrode may be used for a photoelectricconversion layer of a photoelectric conversion device, and the activematerial layer may be used for an antireflective film.

This embodiment can be implemented in appropriate combination with anyof the other embodiments and examples.

Embodiment 2

In this embodiment, an electrode according to an embodiment of thepresent invention, which is partly different from the electrodedescribed in Embodiment 1, will be described with reference to thedrawings.

In an electrode 200 described in this embodiment, the shape of grapheneprovided in the electrode is different from the electrode 100 describedin Embodiment 1. In this embodiment, as numerals using for describingthe electrode 200, those used for describing the electrode 100 are usedas appropriate.

FIGS. 10A and 10B are schematic cross-sectional views of the electrode200 described in this embodiment. The electrode 200 includes a substrate101, a conductive layer 103 provided over the substrate 101, and anactive material layer 108 provided over the conductive layer 103, likethe electrode 100 described in Embodiment 1.

The active material layer 108 includes an active material including aplurality of whiskers including a region 107 a and a region 107 b, andgraphene 116 including a first region 113 and a second region 115 andbeing in contact with the active material including a plurality ofwhiskers like the electrode 100 described in Embodiment 1.

The region 107 a is provided in contact with the conductive layer 103and the region 107 b projects from the region 107 a and provided to bedispersed at random. Therefore, the active material layer 108 has a finesurface structure reflecting the shapes of the active material includinga plurality of whiskers.

In a manner similar to that of the electrode 100, a mixed layer 105 maybe provided in a part of or the whole of a surface layer of theconductive layer 103 of the electrode 200 by reaction with the activematerial layer 108 (in particular, the active material including aplurality of whiskers).

In a manner similar to that of the electrode 100, in the active materiallayer 108 of the electrode 200, the active material including aplurality of whiskers preferably includes a core 109 which has astructure having crystallinity and an outer shell 111 which has anamorphous structure.

The structure having crystallinity has an excellent conductivity and ionmobility and has a feature of high speeds of occlusion and release ofions per unit mass. The amorphous structure has a feature of resistanceto change in volume due to occlusion and release of ions (e.g., stresscaused by change in volume is relieved).

Therefore, by using the electrode 200 including the active materialincluding a plurality of whiskers including the core 109 and the outershell 111, output characteristics are improved, and a power storagedevice with high charge/discharge capacity and improved cyclecharacteristics can be manufactured.

In the active material layer 108 of the electrode 200, the structures ofthe core 109 and the outer shell 111 are similar to those of theelectrode 100.

Further, in the electrode 200, the details of the active materialincluding a plurality of whiskers are the same as the electrode 100. Forexample, the details of the core and the outer shell, and a longitudinalcross-sectional shape and a transverse cross-sectional shape of theelectrode 200 and the like are similar to those of the electrode 100.

As illustrated in FIG. 10A, part of the region 107 a (a region otherthan the region where the conductive layer 103 is in contact with thecore 109) may have an amorphous structure like the outer shell 111.Further, the region 107 a may include a structure having crystallinity.Furthermore, the region 107 a may include one or both of the materialsof the conductive layer 103 and a material of the mixed layer 105.

Moreover, as illustrated in FIG. 10B, a portion of the region 107 awhich is in contact with the conductive layer 103 may have a structurehaving crystallinity like the core 109. Further, the region 107 a mayinclude an amorphous structure. Furthermore, the region 107 a mayinclude one or both of the materials of the conductive layer 103 and thematerial of the mixed layer 105.

In the case where the electrode 200 is the mode illustrated in FIG. 10A,adhesion between the conductive layer 103 and the region 107 a is higherthan in the case where the electrode 100 is the mode illustrated in FIG.10B. Therefore, pulverization and separation of the electrode 200 (inparticular, an active material including a whisker) due to repeatingcharge/discharge cycles can be prevented and a power storage devicehaving much higher cycle characteristics can be manufactured.

Alternatively, in the case where the electrode 200 is the modeillustrated in FIG. 10B, as the whole electrode 200, the conductivitycan be increased more than in the case where the electrode 200 is themode illustrated in FIG. 10A. Therefore, a power storage device withmuch higher output characteristics can be manufactured.

In the electrode 200, the surface area of the active material layer 108is larger than that of a plate-like active material layer due to theprojecting region 107 b. Moreover, since the graphene 116 is included inthe active material layer 108, the surface area of the active materiallayer 108 is much larger. That is, in the case where the electrode 200is included in a power storage device, high speed charge/discharge canbe performed, so that a power storage device with much highercharge/discharge capacity can be manufactured.

In the electrode 200, the active material layer 108 includes thegraphene 116 which is in contact with the active material including aplurality of whiskers and the graphene 116 includes the first region 113and the second region 115. The first region 113 covers each of theactive materials including a plurality of whiskers. In detail, the firstregion 113 also covers the surface of the region 107 a as well as thesurface of the active material in the projecting region 107 b. Further,the thickness of the first region 113 is not uniform in the whole areaand may be uneven.

A second region 115 is provided between a first region 113 at a top ofone of active materials and another or other first regions 113 at a sideor a top of an active material. In addition, the first region 113 is incontact with the second region 115 in any whole range of the activematerial layer 108, so that in a plan view of the active material layer108 (not illustrated), the graphene 116 is provided to spreadcontinuously over the active material including a plurality of whiskers.In other words, the graphene 116 is spread uniformly in the planardirection of the active material layer 108 (the active materialincluding a plurality of whiskers) and in contact with the activematerial including a plurality of whiskers. Note that in thisspecification, the top refers to a region of an active materialincluding a whisker including at least a vertex and a top surface in theregion 107 b. That is, the top includes at least a region including aside of the active material including a whisker. Further, the thicknessof the second region 115 is not uniform in the whole area and may beuneven.

Note that in the graphene 116 of the electrode 200, the interfacebetween the first region 113 and the second region 115 is not clear. InFIGS. 10A and 10B, in a region where the first region 113 is bonded tothe second region 115, the first region 113 and the second region 115 isindividually illustrated by dotted lines for clarity.

As illustrated in dashed dotted lines in FIGS. 10A and 10B, the secondregion 115 in the graphene 116 may be provided between a first region113 at a side of one of the active materials including a whisker and afirst region 113 at a surface of the region 107 a.

Further, the active material layer 108 may have a structure in which anoxide film is provided between the first region 113 and the activematerial including a plurality of whiskers. However, in the terms ofconductivity of the electrode 200, the oxide film is preferably notprovided.

The graphene 116 is highly flexible and in contact with the activematerial including a plurality of whiskers, so that pulverization andseparation of the active material including a whisker due to repeatingcharge/discharge cycles can be prevented. Moreover, the graphene 116 hashigh mechanical strength; therefore, bending and collapsing (alsopulverization and separation) of the active material including a whiskerby physical impact in the electrode 200 including the graphene 116 canbe prevented. Therefore, by using the electrode 200, high speedcharge/discharge can be performed, and a power storage device withhigher charge/discharge capacity can be manufactured.

In the electrode 200, the graphene 116 is spread uniformly in the planardirection of the active material layer 108 (the active materialincluding a plurality of whiskers) and in contact with the activematerial including a plurality of whiskers, whereby a region where thegraphene 116 is in contact with the active material including aplurality of whiskers is large, so that decrease of the conductivity ofthe active material layer can be suppressed. Therefore, by using theelectrode 200, a decrease in charge/discharge capacity by physicalimpact or by repetition of charge/discharge cycles can be suppressed, sothat a power storage device having higher cycle characteristics can bemanufactured.

In addition, the graphene 116 has a capacity so that ions are occluded,and the capacity of the graphene 116 is increased or decreased dependingon the shape (area) of the provided graphene 116. For example, in thecase where the area is spread uniformly in the planar direction of theactive material layer 108 (see FIGS. 10A and 10B), the area and thecapacity of the graphene 116 in the plan view of the active materiallayer 108 are larger than those in the case where holes (also referredto as spaces) at intervals in the active material layer 108 are formed(see FIGS. 1A and 1B). Further, the capacity of the graphene 116 isincreased or decreased also depending on its thickness. The thickness ofthe graphene 116 in the case where the graphene 116 is spread uniformlyin the planar direction of the active material layer 108 (the activematerial including a plurality of whiskers) (FIGS. 10A and 10B) isthicker than that in the case where holes in the active material layer108 are formed. That is, in the case where the graphene 116 is spreaduniformly in the planar direction of the active material layer 108 (theactive material including a plurality of whiskers), a capacity of theelectrode is increased by at least the capacity of the graphene 116.Therefore, by using the electrode 200, a power storage device withhigher charge/discharge capacity can be manufactured.

Further, in the electrode 200, it can be said that the graphene 116 tiesthe active material including a plurality of whiskers. That is, thegraphene 116 also functions as a binder, and the electrode 200 is formedwithout a known conduction auxiliary agent (such as acetylene black) ora binder. As a result, the electrode 200 can increase the volume orweight proportion of the active material layer 108 in the electrode, sothat the electrode 200 functions as an electrode with high capacity.Therefore, using the electrode 200, a power storage device with highercharge/discharge capacity can be manufactured.

Further, the graphene 116 also has high heat resistance, and theconcentration of moisture in the electrode 200 can be reduced by heattreatment. Moreover, in the case where the electrode 200 is included inthe power storage device, the graphene 116 is less capability ofabsorbing an electrolyte, so that deformation or breakdown of theelectrode 200 due to swelling of the graphene 116 hardly occurs.

The electrode 200 can be manufactured using a manufacturing method whichis similar to that of the electrode 100 described in Embodiment 1.

Note that the electrode described in this embodiment may be used foranother application. For example, the active material layer may be usedfor a photoelectric conversion layer of a photoelectric conversiondevice, and the active material layer may be used for an antireflectivefilm.

This embodiment can be implemented in appropriate combination with anyof the other embodiments and examples.

Embodiment 3

In this embodiment, a power storage device according to one embodimentof the present invention will be described.

The power storage device according to one embodiment of the presentinvention includes at least a positive electrode, a negative electrode,a separator, and an electrolyte solution, and the electrode described inEmbodiment 1 is included in the negative electrode.

The electrolyte is a nonaqueous solution containing an electrolyte saltor a solution containing an electrolyte salt. Any electrolyte salt canbe used as the electrolyte salt as long as it contains carrier ions suchas alkali metal ions, alkaline earth metal ions, beryllium ions, ormagnesium ions. Examples of the alkali metal ions include lithium ions,sodium ions, and potassium ions. Examples of the alkaline earth metalions include calcium ions, strontium ions, and barium ions. In thisembodiment, an electrolyte salt containing lithium ions (hereinafterreferred to as a lithium-containing electrolyte salt) is used as theelectrolyte salt.

With the above structure, a lithium-ion secondary battery or alithium-ion capacitor can be formed.

Here, a lithium-ion secondary battery will be described with referenceto drawings.

FIG. 3A illustrates a structural example of a power storage device 300.FIG. 3B is a cross-sectional view along dashed dotted line X-Y in FIG.3A.

The power storage device 300 in FIG. 3A includes a power storage cell304 in an exterior member 302. The power storage device 300 furtherincludes terminal portions 306 and 308 which are connected to the powerstorage cell 304. For the exterior member 302, a laminate film, apolymer film, a metal film, a metal case, a plastic case, or the likecan be used.

As illustrated in FIG. 3B, the power storage cell 304 includes anegative electrode 310, a positive electrode 312, a separator 314provided between the negative electrode 310 and the positive electrode312, and an electrolyte 316 with which a portion almost surrounded withthe exterior member 302 is filled.

The negative electrode 310 includes a negative electrode currentcollector 315 and a negative electrode active material layer 317. Thenegative electrode active material layer 317 is formed on one or both ofthe surfaces of the negative electrode current collector 315. Further,the negative electrode current collector 315 is connected to theterminal portion 308, and the terminal portion 308 partly projectsoutside the exterior material 302.

The positive electrode 312 includes the positive electrode currentcollector 318 and a positive electrode active material layer 320. Thepositive electrode active material layer 320 is formed on one surface oropposite surfaces of the positive electrode current collector 318.Further, the positive electrode 312 may include a binder and aconductive additive besides the positive electrode current collector 318and the positive electrode active material layer 320. A positiveelectrode current collector 318 is connected to the terminal portion306. Further, the terminal portions 306 and 308 each partly extendoutside the exterior member 302.

Although a sealed thin power storage device is described as the powerstorage device 300 in this embodiment, the external shape of the powerstorage device 300 is not limited thereto. A power storage device havingany of a variety of shapes, such as a button power storage device, acylindrical power storage device, or a rectangular power storage device,can be used as the power storage device 300. Further, although thestructure where the positive electrode, the negative electrode, and theseparator are stacked is described in this embodiment, a structure wherethe positive electrode, the negative electrode, and the separator arerolled may be employed.

For the positive electrode current collector 318, a conductive materialsuch as aluminum or stainless steel which is processed into a foilshape, a plate shape, a net shape, or the like can be used.Alternatively, a conductive layer provided by deposition separately on asubstrate and then separated from the substrate can be used as thepositive electrode current collector 318.

The positive electrode active material layer 320 can be formed using anyof LiFeO₂, LiCoO₂, LiNiO₂, LiMn₂O₄, LiFePO₄, LiCoPO₄, LiNiPO₄, LiMnPO₄,V₂O₅, Cr₂O₅, MnO₂, and other lithium compounds as a material. Note thatwhen carrier ions are alkali metal ions other than lithium ions,alkaline earth metal ions, beryllium ions, or magnesium ions, thepositive electrode active material layer 320 can be formed using,instead of lithium in the above lithium compounds, an alkali metal(e.g., sodium or potassium), an alkaline earth metal (e.g., calcium,strontium, or barium), beryllium, or magnesium.

The positive electrode active material layer 320 is formed over thepositive electrode current collector 318 by a coating method or aphysical vapor deposition method (e.g., a sputtering method), wherebythe positive electrode 312 can be formed. In the case where a coatingmethod is employed, the positive electrode active material layer 320 isformed in such a manner that a paste in which a conductive additive (forexample, acetylene black (AB), a binder (e.g., polyvinylidene fluoride(PVDF))), or the like is mixed with any of the above materials for thepositive electrode active material layer 320 is applied to the positiveelectrode current collector 318 and dried. In this case, the positiveelectrode active material layer 320 is preferably molded by applyingpressure as needed.

Note that as the conductive additive, any electron-conductive materialcan be used as long as it does not cause a chemical change in the powerstorage device. For example, a carbon-based material such as graphite orcarbon fibers; a metal material such as copper, nickel, aluminum, orsilver; or a powder or fiber of a mixture thereof can be used.

As the binder, polysaccharides such as starch, carboxymethyl cellulose,hydroxypropyl cellulose, regenerated cellulose, and diacetyl cellulose;vinyl polymers such as polyvinyl chloride, polyethylene, polypropylene,polyvinyl alcohol, polyvinyl pyrrolidone, polytetrafluoroethylene,polyvinylidene fluoride, ethylene-propylene-diene monomer (EPDM) rubber,sulfonated EPDM rubber, styrene-butadiene rubber, butadiene rubber, andfluorine rubber; polyether such as polyethylene oxide; and the like canbe given.

The positive electrode active material layer 320 may be formed using apaste of a mixture of the positive electrode active material andgraphene or multilayer graphene instead of a conductive auxiliary agentand a binder. Note that an alkali metal such as potassium may be addedto the graphene or the multilayer graphene. Further, the graphene andthe multilayer graphene can be obtained by producing the graphene oxidethrough the Hummers method as described in Embodiment 1 and performingreduction treatment.

The use of graphene or multilayer graphene instead of a conductiveadditive and a binder leads to a reduction in amount of the conductiveadditive and the binder in the positive electrode 312. In other words,the weight of the positive electrode 312 can be reduced; accordingly,the charge/discharge capacity of the lithium-ion secondary battery perunit weight of the electrode can be increased.

Strictly speaking, the term “active material” refers only to a materialthat relates to intercalation and deintercalation of ions functioning ascarriers. In this specification, however, in the case of employing acoating method to form the positive electrode active material layer 320,for the sake of convenience, the material of the positive electrodeactive material layer 320, that is, a substance that is actually a“positive electrode active material,” a conductive additive, a binder,and the like are collectively referred to as the positive electrodeactive material layer 320.

The electrode 100 described in Embodiment 1 or the electrode 200described in Embodiment 2 can be applied to the negative electrode 310.That is, in the negative electrode 310, the negative electrode currentcollector 315 corresponds to one of or both the conductive layer 103 andthe mixed layer 105, which are described in Embodiment 1 or Embodiment2, and the negative electrode active material layer 317 corresponds tothe active material layer 108, which is described in Embodiment 1 orEmbodiment 2. Note that in the electrode 100 illustrated in FIGS. 1A and1B or the electrode 200 illustrated in FIGS. 10A and 10B, the activematerial layer 108 is formed on only one surface of the conductive layer103 which functions as the current collector; however, the structure isnot limited thereto, and the active material layer 108 may be formed onboth surfaces of the conductive layer 103. For example, when the activematerial layer is formed using a silicon semiconductor while thenegative electrode current collector 315 is held by a frame-likesusceptor in an LPCVD apparatus, the active material layers can beformed on both the surfaces of the negative electrode current collector315 at the same time. As a result, the number of manufacturing steps canbe reduced in the case where both the surfaces of the negative electrodecurrent collector 315 are used for formation of the electrode.

The negative electrode active material layer 317 may be predoped withlithium. Predoping with lithium may be performed in such a manner that alithium layer is formed on a surface of the negative electrode activematerial layer 317 by a sputtering method. Alternatively, lithium foilis provided on the surface of the negative electrode active materiallayer 317, whereby the negative electrode active material layer 317 canbe predoped with lithium.

The electrolyte 316 is a nonaqueous solution containing an electrolytesalt or a solution containing an electrolyte salt. Particularly in alithium-ion secondary battery, a lithium-containing electrolyte saltwhich is carrier ions and comprises lithium ions can transfer and stablyexist is used. Typical examples of the electrolyte salt include lithiumsalts such as LiClO₄, LiAsF₆, LiBF₄, LiPF₆, and Li(C₂F₅SO₂)₂N. Note thatwhen carrier ions are alkali metal ions other than lithium ions oralkaline earth metal ions, alkali metal salt (e.g., sodium salt orpotassium salt), alkaline earth metal salt (e.g., calcium salt,strontium salt or barium salt), beryllium salt, magnesium salt, or thelike can be used as a solute of the electrolyte 316.

The electrolyte 316 is preferably a nonaqueous solution containing anelectrolyte salt. That is, as a solvent of the electrolyte 316, anaprotic organic solvent is preferably used. Examples of the aproticorganic solvent include ethylene carbonate, propylene carbonate,dimethyl carbonate, diethyl carbonate, γ-butyrolactone, acetonitrile,dimethoxyethane, and tetrahydrofuran, and one or more of these materialscan be used. Alternatively, as the aprotic organic solvent, one ionicliquid or a plurality of ionic liquids may be used. Owing tonon-flammability and non-volatility of an ionic liquid, it is possibleto suppress explosion, inflammation, and the like of the power storagedevice 300 at the time when the internal temperature of the powerstorage device 300 rises, resulting in improvement in safety.

When a gelled high-molecular material containing an electrolyte salt isused as the electrolyte 316, safety against liquid leakage and the likeis improved and the power storage device 300 can be thinner and morelightweight. Examples of the gelled high-molecular material include asilicon gel, an acrylic gel, an acrylonitrile gel, polyethylene oxide,polypropylene oxide, and a fluorine-based polymer.

As the electrolyte 316, a solid electrolyte such as Li₃PO₄ can be used.

As the separator 314, an insulating porous material is used. Forexample, paper; nonwoven fabric; a glass fiber; ceramics; a syntheticfiber containing nylon (polyamide), vinylon (polyvinyl alcohol basedfiber), polyester, acrylic, polyolefin, or polyurethane; or the like maybe used. Note that a material which does not dissolve in the electrolyte316 needs to be selected.

A lithium-ion secondary battery has a small memory effect, a high energydensity, and a high charge/discharge capacity. In addition, the outputvoltage of the lithium ion battery is high. Thus, it is possible toreduce the size and weight of the lithium-ion battery. Further, thelithium ion battery does not easily deteriorate due to repeatedcharge/discharge cycles and can be used for a long time, leading to areduction in cost.

In the case where the power storage device according to one embodimentof the present invention is a lithium-ion capacitor, instead of thepositive electrode active material layer 320, a material capable ofreversibly occluding and releasing one of or both lithium ions andanions may be used. Examples of the material include active carbon,graphite, a conductive high molecule, and a polyacene organicsemiconductor (PAS).

High adhesion between a current collector and an active material layerof both positive electrode and negative electrode in a power storagedevice according to one embodiment of the present invention allows anelectrode to be bended. Thus, the power storage device can be flexible.

Note that this embodiment can be implemented in appropriate combinationwith any of the structures of the other embodiments and example.

Embodiment 4

The power storage device according to one embodiment of the presentinvention can be used for power supplies of a variety of electricappliances which can be operated with power.

Specific examples of electric appliances each utilizing the powerstorage device according to one embodiment of the present invention areas follows: display devices, lighting devices, desktop personalcomputers and laptop personal computers, image reproduction deviceswhich reproduce still images and moving images stored in recording mediasuch as digital versatile discs (DVDs), mobile phones, portable gamemachines, portable information terminals, e-book readers, video cameras,digital still cameras, high-frequency heating appliances such asmicrowave ovens, electric rice cookers, electric washing machines,air-conditioning systems such as air conditioners, electricrefrigerators, electric freezers, electric refrigerator-freezers,freezers for preserving DNA, and medical electrical equipment such asdialyzers. In addition, moving objects driven by electric motors usingpower from power storage devices are also included in the category ofelectric appliances. Examples of the moving objects include electricvehicles, hybrid vehicles each including both an internal-combustionengine and an electric motor, and motorized bicycles includingmotor-assisted bicycles.

In the electric appliances, the power storage device according to oneembodiment of the present invention can be used as a power storagedevice for supplying enough power for almost the whole power consumption(referred to as a main power supply). Alternatively, in the electricappliances, the power storage device according to one embodiment of thepresent invention can be used as a power storage device which can supplypower to the electric appliances when the supply of power from the mainpower supply or a commercial power supply is stopped (such a powerstorage device is referred to as an uninterruptible power supply). Stillalternatively, in the electric appliances, the power storage deviceaccording to one embodiment of the present invention can be used as apower storage device for supplying power to the electric appliances atthe same time as the power supply from the main power supply or acommercial power supply (such a power storage device is referred to asan auxiliary power supply).

FIG. 4 illustrates specific structures of the electric appliances. InFIG. 4, a display device 5000 is an example of an electric applianceincluding a power storage device 5004 according to one embodiment of thepresent invention. Specifically, the display device 5000 corresponds toa display device for TV broadcast reception and includes a housing 5001,a display portion 5002, speaker portions 5003, and the power storagedevice 5004. The power storage device 5004 according to one embodimentof the present invention is provided in the housing 5001. The displaydevice 5000 can receive power from a commercial power supply.Alternatively, the display device 5000 can use power stored in the powerstorage device 5004. Thus, the display device 5000 can be operated withthe use of the power storage device 5004 according to one embodiment ofthe present invention as an uninterruptible power supply even when powercannot be supplied from a commercial power supply due to power failureor the like.

A semiconductor display device such as a liquid crystal display device,a light-emitting device in which a light-emitting element such as anorganic EL element is provided in each pixel, an electrophoresis displaydevice, a digital micromirror device (DMD), a plasma display panel(PDP), or a field emission display (FED) can be used for the displayportion 5002.

Note that the display device includes, in its category, all ofinformation display devices for personal computers, advertisementdisplays, and the like besides TV broadcast reception.

In FIG. 4, an installation lighting device 5100 is an example of anelectric appliance including a power storage device 5103 according toone embodiment of the present invention. Specifically, the lightingdevice 5100 includes a housing 5101, a light source 5102, and a powerstorage device 5103. Although FIG. 4 illustrates the case where thepower storage device 5103 is provided in a ceiling 5104 on which thehousing 5101 and the light source 5102 are installed, the power storagedevice 5103 may be provided in the housing 5101. The lighting device5100 can receive power from a commercial power supply. Alternatively,the lighting device 5100 can use power stored in the power storagedevice 5103. Thus, the lighting device 5100 can be operated with the useof the power storage device 5103 according to one embodiment of thepresent invention as an uninterruptible power supply even when powercannot be supplied from a commercial power supply due to power failureor the like.

Note that although the installation lighting device 5100 provided in theceiling 5104 is illustrated in FIG. 4 as an example, the power storagedevice according to one embodiment of the present invention can be usedin an installation lighting device provided in, for example, a wall5105, a floor 5106, a window 5107, or the like other than the ceiling5104. Alternatively, the power storage device can be used in a tabletoplighting device or the like.

As the light source 5102, an artificial light source which emits lightartificially by using power can be used. Specifically, discharge lampssuch as an incandescent lamp and a fluorescent lamp, and light-emittingelements such as an LED and an organic EL element are given as examplesof the artificial light source.

In FIG. 4, an air conditioner including an indoor unit 5200 and anoutdoor unit 5204 is an example of an electric appliance including apower storage device 5203 according to one embodiment of the invention.Specifically, the indoor unit 5200 includes a housing 5201, an airoutlet 5202, and a power storage device 5203. Although FIG. 4illustrates the case where the power storage device 5203 is provided inthe indoor unit 5200, the power storage device 5203 may be provided inthe outdoor unit 5204. Alternatively, the power storage devices 5203 maybe provided in both the indoor unit 5200 and the outdoor unit 5204. Theair conditioner can receive power from a commercial power supply.Alternatively, the air conditioner can use power stored in the powerstorage device 5203. Particularly in the case where the power storagedevices 5203 are provided in both the indoor unit 5200 and the outdoorunit 5204, the air conditioner can be operated with the use of the powerstorage device 5203 according to one embodiment of the present inventionas an uninterruptible power supply even when power cannot be suppliedfrom a commercial power supply due to power failure or the like.

Note that although the split-type air conditioner including the indoorunit and the outdoor unit is illustrated in FIG. 4 as an example, thepower storage device according to one embodiment of the presentinvention can be used in an air conditioner in which the functions of anindoor unit and an outdoor unit are integrated in one housing.

In FIG. 4, an electric refrigerator-freezer 5300 is an example of anelectric appliance including a power storage device 5304 according toone embodiment of the present invention. Specifically, the electricrefrigerator-freezer 5300 includes a housing 5301, a door for arefrigerator 5302, a door for a freezer 5303, and the power storagedevice 5304. The power storage device 5304 is provided in the housing5301 in FIG. 4. The electric refrigerator-freezer 5300 can receive powerfrom a commercial power supply. Alternatively, the electricrefrigerator-freezer 5300 can use power stored in the power storagedevice 5304. Thus, the electric refrigerator-freezer 5300 can beoperated with the use of the power storage device 5304 according to oneembodiment of the present invention as an uninterruptible power supplyeven when power cannot be supplied from a commercial power supply due topower failure or the like.

Note that among the electric appliances described above, ahigh-frequency heating apparatus such as a microwave oven and anelectric appliance such as an electric rice cooker require high power ina short time. The tripping of a breaker of a commercial power supply inuse of an electric appliance can be prevented by using the power storagedevice according to one embodiment of the present invention as anauxiliary power supply for supplying power which cannot be suppliedenough by a commercial power supply.

In addition, in a time period when electric appliances are not used,particularly when the proportion of the amount of power which isactually used to the total amount of power which can be supplied from acommercial power supply source (such a proportion referred to as a usagerate of power) is low, power can be stored in the power storage device,whereby the usage rate of power can be reduced in a time period when theelectric appliances are used. For example, in the case of the electricrefrigerator-freezer 5300, power can be stored in the power storagedevice 5304 in night time when the temperature is low and the door for arefrigerator 5302 and the door for a freezer 5303 are not often openedor closed. On the other hand, in daytime when the temperature is highand the door for a refrigerator 5302 and the door for a freezer 5303 arefrequently opened and closed, the power storage device 5304 is used asan auxiliary power supply; thus, the usage rate of power in daytime canbe reduced.

This embodiment can be implemented in appropriate combination with anyof the other embodiments and example.

Example 1

In this example, evaluation results of an electrode which is formed fora power storage device according to one embodiment of the presentinvention will be described. Note that descriptions in this example willbe given using FIGS. 1A and 1B, FIG. 2, and reference numerals in FIGS.1A and 1B and FIG. 2.

In this example, a titanium sheet (the purity: 99.5%, the thickness: 0.1mm) was used as the conductive layer 103. Therefore, the substrate 101was not used in this example. Note that the titanium foil was immersedin a 0.5% hydrofluoric acid solution for 10 minutes, whereby the surfacethereof was cleaned.

Next, by using silicon for an active material including a plurality ofwhiskers, the active material including a plurality of whiskersincluding a core 109 which is silicon having a structure havingcrystallinity (hereinafter, referred to as crystalline silicon) and anouter shell 111 which is silicon having an amorphous structure(hereinafter, referred to as amorphous silicon) was formed over thetitanium sheet. Note that in this example, as illustrated in FIG. 1A,the active material including a plurality of whiskers whose part of theinterface with the titanium sheet was formed of amorphous silicon likethe outer shell 111 was formed.

Specifically, the active material including a plurality of whiskers wasformed over the titanium sheet by an LPCVD method. The LPCVD method wasperformed in a reaction chamber into which silane gas (SiH₄ gas) andnitrogen gas (N₂ gas) were each introduced as a source gas at a flowrate of 300 sccm and in which the pressure was 150 Pa and in which thetemperature was 550° C. The reaction chamber made of quartz was used.When the temperature of the titanium sheet was increased, an argon gaswas introduced into the reaction chamber.

Next, a solution in which graphene oxide had been dispersed (correspondsto the graphene oxide solution 202 in FIG. 2) was prepared. The solutioncan be manufactured by forming graphite oxide with a Hummers method andapplying ultrasonic vibration to the graphite oxide, as described inEmbodiment 1. However, in this example, a graphene oxide aqueoussolution (concentration: 0.275 mg/ml, flake size: 0.5 μm to 5 μm) whichis commercially available (Graphene Supermarket) was used.

Then, by employing the electrophoresis method described in Embodiment 1,graphene oxide was formed around the active material including aplurality of whiskers. Specifically, a titanium sheet with the activematerial including a plurality of whiskers was immersed in the grapheneoxide solution 202, and a stainless steel plate was immersed therein asan electrode (see FIG. 2). Here, the distance between the titanium sheetand the stainless steel plate was 1 cm. Then, with the sheet used as ananode and the stainless steel plate as a cathode, a voltage of 10 V wasapplied between the anode and the cathode for 15 minutes. The amount ofcharge flowing during the 15 minutes was 0.223 C. Note that the activematerial including a plurality of whiskers with the titanium sheetcorresponds to the formation subject 203 in FIG. 2.

After that, the titanium sheet was taken out of the solution, dried, andthen heated at 300° C. in a vacuum (0.1 Pa or less) for 10 hours. Asample formed through the above steps was used as Electrode A.

A part of a surface of Electrode A was observed. FIGS. 5A and 5B areplanar scanning electron microscope (SEM) image of the part. Themagnifications of the images in FIGS. 5A and 5B are 1000 times and 3000times, respectively.

As seen in FIGS. 5A and 5B, Electrode A has an active material includinga plurality of whiskers over the titanium sheet. The tops of some of theactive materials including a whisker are curved. The longitudinaldirections of the active materials including a whisker are not the same.

In addition, peaks of a D band and a G band, which are derived fromgraphene, were observed in any portion of the active materials includinga whisker measured by Raman spectroscopy. This indicates thatsubstantially the surface of the active material including a pluralityof whiskers is probably covered with graphene (corresponds to the firstregion 113 in FIGS. 1A and 1B).

In Electrode A, graphene which corresponds to the second region 115 (theelectrode 100) in FIGS. 1A and 1B was found. In particular, graphene wasobserved noticeably in a region below a dotted line portion in FIG. 5A.In the region, the graphene was formed between a first region 113 at aside of one of the active materials including a whisker and another orother first regions 113 at a side of an active material.

Further, the second region 115 was dotted in a range of the activematerial including a plurality of whiskers. That is, there was also anactive material which does not include the second region 115. Therefore,in Electrode A, holes (also referred to as spaces) were found atintervals in the active material layer 108.

Next, cross-sectional transmission electron microscope (TEM) image ofthe part of Electrode A is shown (see FIG. 6A). The magnification of theimage in FIG. 6A is 20500 times.

As seen in FIG. 6A, in Electrode A, an active material including aplurality of whiskers which corresponds to the region 107 a and theregion 107 b in FIG. 1A is formed over the titanium sheet (theconductive layer 103). Note that in FIGS. 6A and 6B, a region whichcorresponds to the region 107 a of FIG. 1A is represented as a region107 a. In addition, a carbon film which had been deposited byevaporation treatment in the observation was formed around the activematerial including a plurality of whiskers.

Moreover, it is found that the region 107 a in FIG. 6A has an amorphousstructure. The active material including a plurality of whiskersincludes a core having a structure having crystallinity (corresponds tothe core 109 a in FIG. 1A) and an outer shell having an amorphousstructure (corresponds to the outer shell 111 a in FIG. 1A). Note thatthe differences of a structure having crystallinity and an amorphousstructure can be expressed by a contrast in FIG. 6A.

FIG. 6B shows an enlarged view of a dotted rectangle in FIG. 6A. Notethat the magnification of FIG. 6B is 2050000 times. FIG. 6B shows thatgraphene which corresponds to the first region 113 in FIGS. 1A and 1Bwas formed around the active materials including a whisker in ElectrodeA. The thickness of the graphene was about from 2 nm to 3 nm. Note thatas seen in FIG. 6B, a natural oxide film is formed between the activematerial including a whisker and the graphene.

Moreover, FIG. 7A is a cross-sectional TEM image of a different part ofElectrode A from the part in FIG. 6A. In FIG. 7A, an active materialincluding a plurality of whiskers was also formed over the titaniumsheet (the conductive layer 103) like in FIG. 6A. Further, the activematerial including a whisker includes a core having a structure havingcrystallinity (corresponds to the core 109 a in FIG. 1A) and an outershell having an amorphous structure (corresponds to the outer shell 111a in FIG. 1A).

FIG. 7B shows an enlarged view of a region at a point X in FIG. 7A. Notethat the magnification of FIG. 7B is 2050000 times. As seen in FIG. 7B,graphene which corresponds to the second region 115 in FIGS. 1A and 1Bis formed in Electrode A. The thickness of the graphene was about from4.6 nm to 5.6 nm, which indicates that the graphene is thicker than thegraphene (the first region 113) formed around the active materialincluding a plurality of whiskers shown in FIG. 6B. In addition, acarbon film which had been deposited by evaporation treatment in theobservation was formed over the second region 115. Moreover, holes wereobserved between the active materials including a plurality of whiskersand the second region 115. Note that a black region in FIG. 7B is aprocessed residue attached in the observation.

Further, in Electrode A, graphene which corresponds to the second region115 in FIGS. 10A and 10B (the electrode 200) was observed. Inparticular, the graphene was observed noticeably in a region above adotted line portion in FIG. 5A. The graphene was spread continuously andprovided over the active material including a plurality of whiskers inthe region. In other words, the graphene 116 is spread uniformly in theplanar direction of the active material layer 108 (the active materialincluding a plurality of whiskers) and in contact with the activematerial including a plurality of whiskers in the region.

According to this example, in the electrode for the power storage deviceaccording to one embodiment of the present invention, a core which has astructure having crystallinity is provided in the active materialincluding a plurality of whiskers and graphene which has highconductivity (high electron mobility) is provided between the activematerials including a plurality of whiskers. Therefore, it can be saidthat the electrode for the power storage device according to oneembodiment of the present invention has excellent electriccharacteristics.

In addition, according to this example, in the electrode for the powerstorage device according to one embodiment of the present invention, anactive material having an amorphous structure and an outer shell havingan amorphous structure are provided between a conductive layer and anactive material layer (in particular, an active material including aplurality of whiskers). Therefore, it can be said that in the electrodefor the power storage device according to one embodiment of the presentinvention, even if the volume of an active material is changed due toocclusion and release of ions, the structure of the electrode is noteasily damaged, for example, pulverization and separation.

Example 2

In this example, electric characteristics of a power storage devicewhich is an embodiment of the present invention will be described.Specifically, as the power storage device, a lithium ion secondarybattery was manufactured and evaluated.

A method for manufacturing a coin-type secondary battery is describedbelow with reference to FIG. 8.

As illustrated in FIG. 8, the coin-type secondary battery includes anelectrode 401, a reference electrode 403, a separator 405, anelectrolyte (not shown), a housing 407, and a housing 409. Besides, thecoin-type secondary battery includes a ring-shaped insulator 411, aspacer 413, and a washer 415. As the electrode 401, Electrode A obtainedby the process shown in Example 1 was used. The reference electrode 403includes a reference electrode active material layer 417. The referenceelectrode active material layer 417 was formed using lithium foil. Theseparator 405 was formed using polypropylene. The housing 407, thehousing 409, the spacer 413, and the washer 415 each of which was madeusing stainless steel (SUS) were used. The housing 407 and the housing409 electrically connect the electrode 401 and the reference electrode403 to the outside.

The electrode 401, the reference electrode 403, and the separator 405were soaked in the electrolyte. Then, as illustrated in FIG. 8, thehousing 407, the electrode 401, the separator 405, the ring-shapedinsulator 411, the reference electrode 403, the spacer 413, the washer415, and the housing 409 were stacked in this order such that thehousing 407 was positioned at the bottom of the stacked components. Thehousing 407 and the housing 409 were crimped with a “coin cell crimper”.In such a manner, the coin-type secondary battery (called SecondaryBattery A) was formed.

The electrolyte in which LiPF₆ was dissolved in a mixed solvent ofethylene carbonate (EC) and diethyl carbonate (DEC) and theconcentration was adjusted to be 1 mol/L was used.

The charge/discharge cycle characteristics of the Secondary Battery Awhich were formed through the above steps was evaluated. FIG. 9A showsthe result of cycle characteristics of the obtained Secondary Battery A.In FIG. 9A, only a discharging curve is shown for simplicity, and thehorizontal axis indicates the number of cycles (unit: time), and thevertical axis indicates discharge capacity (unit: mAh/g). Charging anddischarging were in one cycle, and the one cycle was performed 100times. The first charging rate and the discharging rate were each 0.2 C,and the second or later charging rate and the discharging rate were each0.5 C. Note that the potential range is from 0.03 V to 1.0 V (vs.Li/Li⁺).

Moreover, FIG. 9B shows the charging and discharging curves in thesecond cycle. In FIG. 9B, the solid line shows the charging curve andthe dotted line shows the discharging curve. Further, the horizontalaxis indicates charge/discharge capacity (unit: mAh/g), and the verticalaxis indicates voltage at the time of charging and discharging (unit:V).

From FIGS. 9A and 9B, it can be confirmed that Secondary Electrode Aincluding Electrode A obtained by the process shown in Example 1 cancharge and discharge, and functions as a lithium ion secondary battery.

This application is based on Japanese Patent Application serial no.2011-179958 filed with Japan Patent Office on Aug. 19, 2011, the entirecontents of which are hereby incorporated by reference.

1. An electrode for a power storage device, comprising: a conductivelayer; and an active material layer over the conductive layer, whereinthe active material layer includes graphene, an active material and ahole, wherein the active material has an uneven surface, wherein thegraphene is provided to be attached to the uneven surface of the activematerial, and wherein the hole included in the active material layer issurrounded with the graphene.
 2. The electrode for a power storagedevice, according to claim 1, wherein the active material including aplurality of whiskers includes at least a core and an outer shellprovided to cover the core, wherein the core has a structure havingcrystallinity, and wherein the outer shell has an amorphous structure.3. The electrode for a power storage device, according to claim 1,wherein the active material including a plurality of whiskers comprisessilicon.
 4. The electrode for a power storage device, according to claim1, wherein a material of the conductive layer comprises one selectedfrom the group consisting of titanium, zirconium, hafnium, vanadium,niobium, tantalum, chromium, molybdenum, tungsten, cobalt and nickel. 5.The electrode for a power storage device, according to claim 1, whereina proportion of elements except hydrogen and carbon included in thegraphene is lower than or equal to 15 atomic % or a proportion ofelements except carbon included in the graphene is lower than or equalto 30 atomic %.
 6. A power storage device including the electrode,according to claim
 1. 7. An electronic appliance including the powerstorage device, according to claim
 6. 8. An electrode for a powerstorage device, comprising: a conductive layer; and an active materiallayer over the conductive layer, wherein the active material layerincludes graphene and an active material including a plurality ofwhiskers, wherein the graphene is provided to be attached to a surfaceof the active material including a plurality of whiskers, and whereinthe graphene covers the active material including a plurality ofwhiskers.
 9. The electrode for a power storage device, according toclaim 8, wherein the graphene is provided to be spread continuously overthe active material including a plurality of whiskers in a plan view ofthe active material layer.
 10. The electrode for a power storage device,according to claim 8, wherein the active material including a pluralityof whiskers includes at least a core and an outer shell provided tocover the core, wherein the core comprises a structure havingcrystallinity, and wherein the outer shell comprises an amorphousstructure.
 11. The electrode for a power storage device, according toclaim 8, wherein the active material including a plurality of whiskerscomprises silicon.
 12. The electrode for a power storage device,according to claim 8, wherein a material of the conductive layercomprises one selected from the group consisting of titanium, zirconium,hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten,cobalt and nickel.
 13. The electrode for a power storage device,according to claim 8, wherein a proportion of elements except hydrogenand carbon included in the graphene is lower than or equal to 15 atomic% or a proportion of elements except carbon included in the graphene islower than or equal to 30 atomic %.
 14. A power storage device includingthe electrode, according to claim
 8. 15. An electronic applianceincluding the power storage device, according to claim
 14. 16. Anelectrode for a power storage device comprising: a conductive layer; anactive material layer over the conductive layer, wherein the activematerial layer includes graphene and an active material including afirst whisker and a second whisker; a first graphene which covers theactive material continuously, wherein the first graphene covers thefirst whisker and the second whisker, and the first graphene has asubstantially even thickness; and a second graphene over the firstgraphene and in contact with the first graphene, wherein the secondgraphene is provided between the first whisker and the second whisker,and is connected to the first whisker and the second whisker.
 17. Theelectrode for a power storage device, according to claim 16, wherein thefirst graphene is provided to be spread continuously over the activematerial including the first whisker and the second whisker in a planview of the active material layer.
 18. The electrode for a power storagedevice, according to claim 16, wherein the active material including thefirst whisker and the second whisker includes at least a core and anouter shell provided to cover the core, wherein the core comprises astructure having crystallinity, and wherein the outer shell comprises anamorphous structure.
 19. The electrode for a power storage device,according to claim 16, wherein the active material including the firstwhisker and the second whisker comprises silicon.
 20. The electrode fora power storage device, according to claim 16, wherein a material of theconductive layer comprises one selected from the group consisting oftitanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium,molybdenum, tungsten, cobalt and nickel.
 21. The electrode for a powerstorage device, according to claim 16, wherein a proportion of elementsexcept hydrogen and carbon included in the first graphene or the secondgraphene is lower than or equal to 15 atomic % or a proportion ofelements except carbon included in the first graphene or the secondgraphene is lower than or equal to 30 atomic %.
 22. A power storagedevice including the electrode, according to claim
 16. 23. An electronicappliance including the power storage device, according to claim 22.